The present invention presents an assay for transmissible spongiform encephalopathies in humans and other animals.
Transmissible spongiform encephalopathies (“TSE”) are diseases that can affect humans and other animals. It is characterized by spongy degeneration of the brain. The disease is known as scrapie in sheep and goats. The condition is known as both bovine spongiform encephalopathy or mad cow disease in cattle. There exist human variants of the disease known as Kuru, Creutzfeldt-Jakob Disease (“CJD”), fatal familial insomnia, and Gerstmann-Strausler disease. Deer and elk are known to contract a variant of the disease known as chronic wasting disease. TSE is seen in farmed mink, known as transmissible mink encephalopathy.
At a molecular level, the disease is characterized by deposition of prion protein in brains from TSE-afflicted humans or animals. The prion protein is a normal constituent of brain tissue. In individuals affected by transmissible spongiform encephalopathies, there is conformational change in the protein making it resistant to denaturation by proteolytic enzymes. Since the prion proteins of affected individuals are resistant to most methods of denaturation, most assays for the diseased version of the protein seek to differentiate between the diseased and normal version of the protein. Typical assays for the protein first treat suspected brain tissue with proteolytic enzymes, then seek to identify the prion proteins (usually by polyacrylamide gel electrophoresis followed by western blotting with an antibody specific for both types of prion proteins). In unaffected individuals, there is often no protein available for recognition by the antibodies during western blotting following proteolytic digestion. Because of the ability of diseased prion proteins to resist proteolytic denaturation, they are recognized by the anti-prion antibodies. An alternate approach is to use antibody recognition of the prion protein by Enzyme-Linked ImmunoadSorbent Assay (“ELISA”) using antibodies tagged with enzymes or fluorescent molecules. In either case, a fluorescent or calorimetric signal can be used to conclude the testing. Abnormal Prion has not been detected in sera or cerebrospinal fluid derived from TSE-afflicted individuals.
Current methods of testing for the presence of transmissible spongiform encephalopathies are for the most part conducted post-mortem. This is because an analysis of the prion proteins of the individuals must be done and this is accomplished, by an analysis of brain tissue or other tissues such as tonsil. Thus, such analyses are extremely invasive negating applicability to preclinical diagnosis of TSE. These assays, while useful, suffer from their reliance on brain tissue to provide a diagnostic result. The use of brain material is not feasible for screening. This is especially not suitable when the individual to be tested is a living human or for the purpose only to determine if the individual is a safe blood donor. Therefore there exists a need to develop a test that can provide diagnosis in a living individual with minimal invasion, preferably using sera samples.
An alternate approach has led to a better understanding of the pathogenesis of TSE whereby a wall-less bacterium called Spiroplasma is closely associated with these diseases. The presence of Spiroplasma was initially discovered in an ultrastructural study of a brain biopsy obtained from a 46 year old CJD patient. Since then, the presence of Spiroplasma genes in TSE brain tissues has been shown using molecular techniques including polymerase chain reaction (PCR), Southern blotting and DNA sequence analyses. More recently, a unique Spiroplasma species from TSE-infected brain tissues has been isolated by passage through embryonated eggs into cell-free media. The role of Spiroplasma infection in the pathogenesis of TSE is supported by recent studies that have shown the normal prion isoform to serve as a receptor protein for a bacterium. It is presumed that interaction of Spiroplasma with the prion results in the disease and accumulation of the misfolded prion protein.
The occurrence of a consistent Spiroplasma infection in association with individual TSE cases provided the opportunity to develop a serum test for the disease based upon the presence of antibodies generated against Spiroplasma proteins. Heat shock protein 60 (Hsp60) was chosen because of prior data that showed interaction of bacterial-specific Hsp60 protein on the bacterial surface with the prion receptor. It is noteworthy that Hsp60 is widespread in nature. However, the Hsp60 of bacteria is specific in that there is 70% homology with other bacteria, but a 100% homology among strains of the same genus. There is 50% homology of bacterial Hsp60 with human Hsp60 proteins. The inventor has isolated the Hsp60 gene specific for Spiroplasma, produced recombinant Spiroplasma-specific Hsp60 recombinant protein and shown reactivity with sera from individual TSE cases by using ELISA.
The current methods of testing are also inefficient when applied to large numbers of livestock. Brain or neural material must be taken after slaughter and processed to be assayed. This delay can result in the carcass of the animal being placed into the human or animal food supply before testing can be concluded. Also, these postmortem assays cannot be used to test and produce groups of animals that are free of transmissible spongiform encephalopathies or insure that animals with transmissible spongiform encephalopathy do not co-mingle with disease free animals. The use of brain tissue, in addition to mandating a postmortem test, is also very inconvenient. An assay using a more readily available bodily fluid such as sera or tissue offers a more convenient approach to testing of any animal type for TSE.